Specifications

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Figure 9 - Charging Characteristics of a Nickel-Metal Hydride Battery
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The most preferred method for nickel-based chargers is known as Negative Delta V
(NDV). Through NDV, the microcontroller in the system measures a voltage drop in the
charger. This is a defined voltage signature that occurs when the battery has reached
full capacity. It is the most accurate method when it comes to measuring most nickel-
based chargers.
A specific problem with nickel-metal hydride batteries is that the voltage drop
experienced when the battery has reached full charge is very minute, and the
microcontroller must respond to a drop of 5mV per cell. Because of this extremely low
voltage drop, the charger must include electronic filtering, which would account for noise
and voltage fluctuations within the battery and the charger. Because this method is not
fully reliable, our charger would need to include other monitoring methods in order to
detect when the battery is fully charged.
Other methods that can be combined with the NDV method to detect a full battery
charge include setting a voltage plateau, monitoring the temperature of the battery,
monitoring the change in temperature of the battery, and incorporating a charge timer.
The voltage plateau can be used to shut off the charge when it has reached a certain
voltage level, but this is a rough estimate of the battery‟s charge state. The outside
temperature of the battery can also be used to indicate when a battery is at full charge.
Over charge can occur from this method, however, as the internal temperature of the
battery is warmer than the skin. A more secure method is to monitor the rate of
temperature change over time and to use a set threshold to cut off the charge. If the
threshold rate is never achieved, then at that point the temperature of the battery could